(Please allow me to preface the following with the fact that I have been up nearly 24 hours. I didn't get much sleep the night before either.)

You can use any repository you want. Just make sure you don't upgrade the X servers (i.e. xlibs, xbase, xbase-clients, xserver-common, etc.) or you'll hork the kdrive servers.

Aside from that, I don't see that using any repository in your sources.list will break anything. True, if Dillo, links-hacker, scite, et.al. were built into deb's, it'd be easier to upgrade hdinstalls, but it won't kill you if you have two dillo's. Well, depending on hard drive space and whatnot, that and ensuring which one you have in your path, nah, I don't think it'll make much of a difference one way or another.

First thing I did, and admitedly with an earlier version, was to cut & paste the existing list, switching stable to unstable (or was it vice versa) - Nothing broke

It will only complain if you do something wrong.........The one reason why I like synaptic is that it lists the dependancies needed, package sizes, any conflicts and gives you the chance to consider what you're doing.